This invention concerns a device and method for sealing a package according to the preamble of claim 1 and 6, and 7, respectively.
For many kinds of goods, mainly foodstuffs, the continuous tube package, which is formed and continuously filled and regularly constricted, is the absolutely most rational package type. Hereby the package material at each end is collected into a small string, which is sealed with metal clamps or clips. Its weakness is that end closures, formed at the cut off, demands the use of these metal clamps, which still do not guarantee a tight closure. The clamps, further, comprise an economic drawback by their costs and by complicating material recycling.
It is an object of the invention to eliminate the drawbacks of the prior art and to provide a device and a method according to the above, which allow perfected production of seals for tube shaped packages.
This invention particularly concerns an essentially faster and thereby more economic method of making durable heat sealing of the tube material.
According to a first aspect of the invention this is obtained by using tube material including plural plastics having different melting points and applying ultrasound to the compressed string in a device, which is mainly characterised in that the V-shaped tool parts are arranged on each side of the string and movable in pairs against each other in a transverse direction with respect to the string, that the tool part pairs have a mutual distance of 15-25 mm in the longitudinal direction of the string and that between the pairs there are arranged on the one hand two supporting surfaces for the opposite sides of the string, on the one hand two jaws, which are displaceable against each other for ultrasound hammering the string. In a particularly preferred embodiment, at least one of the jaws is provided with ridges which extend essentially transversally with respect to the string, or in a checked pattern. The jaws may further include a cutting edge tool for cutting the string at the end phase of its closure.
The sealing problems of such tubular packages are thus completely solved through this invention. The filled tube may be end sealed, reliably as well as gas tight, even without the use of additives. According to a second aspect of the invention this is achieved through a method, which is mainly characterised in that the tube is produced from a material including a meltable layer, that the string at the constricting positions are pressed together into a string, and that the string along plural lines is subjected to a high and preferably pulsating pressure of such a magnitude that the meltable layer in the material will become temporarily adhesive. This adhesion will be permanent when the material has been allowed to cool.
It is preferred that the tube is manufactured from a material consisting of several layers having different melting temperatures, whereby the more easily melting layers or the material will provide the adhesion.
Thorough investigation of the process when end sealing tubular packages has revealed that bars of today against a safe and economic working method generally are of two kinds. On the one hand it concerns the difficulty to remove, from the sealing spot, all remains of the contents of the tube, on the other hand about unavoidable variations of the temperature in the different layers of the tube material resulting from heat applied from the outside. Even though the remaining contents being retained in the sealing area seem not to be of any importance, they do present a risk of leakage, hitherto only somewhat being controllable by the application of two metal clamps at each constriction of the tube. According to the invention the impact of the remains to the tightness of the closure is eliminated by the sealing function being distributed to several lines, between which possible remaining contents may be displaced by the pressure at the linear closure positions. It is thereby sufficient if the lines are extending in parallel to each other and transversally with respect to the longitudinal direction of the tube, but if the lines are arranged to form a checked pattern, their effect may be further enhanced so as to solve particularly difficult problem cases. It is further very important that by the arrangement of sealing at plural lines, melting material to achieve the adhesion is collected within the area of the closure and is retained there, which guarantees that the bonding or adhering material does not escape from this area.
Each constriction includes two sealing places, each having to have any of the described line arrangements. Between these places, the lengths of which will amount to about 5-7 mm each, a distance of some millimeters may be left unaffected and soft in order that after cutting it must not comprise a sharp and thereby risky point.
The best result concerning tightness as well as economy is achieved with the method according to the invention by using ultrasound for providing pulsating pressure. Thereby firstly, a maximum pressure is applied so as to transport the remains from the lines and at the same time cold-shape the tube material. Immediately and following that, the sealing is achieved with the aid of the ultra-sound which in a known manner heats the tube material and under a pressure, which may be somewhat longer than the initial pressure, will melt it together within a fraction of a second. At that process the softer layer in the tube material will act as a melt and adhere in lines to the other layers and encloses the remains of the contents of the package between the lines. The lines may be parallel and transverse with respect to the tube or be crossing each other and form a checked pattern. The number must be at least four for every constricting position, preferably more, and they can with advantages be obtained with a correspondingly patterned stamp, working against an anvil. Also the anvil may be patterned in the same way. The anvil may preferably be held at a constant temperature with the aid of a cooling device and a thermostat. The choice of materials of the different layers of the tube must of course follow the demands set by the contents of the package, but it could be mentioned as an example that for food stuffs, has been used advantageously: the combination polyethylene-polypropylene-polyethylene as well as metallocen-polyethylene-polyamide-polyethylene-metallocen. All layers must not consist of a plastics material. Also less expensive material which is coated with a plastic material may be used if this is allowed with respect to the contents of the package. Plastic coated metal foil may be used for light sensitive goods.
Suitable material choices for the stamp and the anvil, for carrying out the method is, polytetrafluorethylene (teflon) or teflon covered aluminum, and for the stamp a metal, preferably titanium. Also the stamp may be held at a constant temperature with the aid of cooling means and a thermostat.
Within the scope of the invention is also that the pressure lines may consist of lines of points and depressions near each other.